The demand for magnetic disk storage capability continues to increase. This has resulted in increased storage density on disks. Increased storage density generally results in a decreased signal level from, for example, a reading head. Therefore, a magnetic head which has increased output may be beneficial.
In the past, magnetic resistance sensors with a spin-valve film structure that have shown a large magnetic resistance effect (spin-valve MR sensors) have been developed in order to increase the output by increasing the magnetic field sensitivity, and thus possibly achieve a further increase in the magnetic recording density, in playback magnetic heads.
Generally, spin-valve MR films consist of a sandwich structure in which two facing ferromagnetic layers are stacked with a non-magnetic spacer layer consisting of a conductive metal sandwiched in between. The magnetization of one of the ferromagnetic layers is fixed in the direction parallel to the signal magnetic field by the exchange-coupling magnetic field with the adjacent anti-ferromagnetic layer, while the magnetization of the other ferromagnetic layer is generally formed into a single magnetic domain by a hard bias method utilizing the magnetic field of a permanent magnet, and rotates freely in accordance with variations in the external magnetic field. The ferromagnetic layer whose direction of magnetization is fixed is called a pinned layer or pinned ferromagnetic layer, and the ferromagnetic layer whose magnetization rotate freely is called a free layer or free ferromagnetic layer.
When the magnetization of the free layer is caused to rotate by the external magnetic field from a magnetic recording medium, etc., the magnetic resistance of the spin-valve film varies according to the angular difference in the direction of magnetization that is generated between the two magnetic layers. Data recorded on a magnetic recording medium such as a magnetic disk, etc., can be recognized by sensing this variation in the magnetic resistance. For example, the principle of such a spin-valve sensor is also disclosed in detail in Japanese Patent Publication No. 8-503336 (PCTIUS93/10782).
Reducing the film thickness of the free layer so that the coercive force of said free layer is lowered is an effective means of increasing the sensitivity of a spin-valve film. However, if the film thickness of the free layer is reduced to a value of 3.0 to 5.0 nm, which is comparable to the mean free path of the conductive electrons, the magnetic resistance variation rate (MR ratio) drops.
Recently, in order to solve this problem, a method in which the MR ratio is increased by forming a non-magnetic metal layer (back layer) so that this metal layer is adjacent to the surface of the free layer on the opposite side from non-magnetic spacer layer, this extending the mean free path of the conductive electrons, has been investigated (for example, in Japanese Patent No. 2744883).
Furthermore, as a separate method, a spin-valve film in which an oxide layer that reflects electrons is stacked in the pinned layer and adjacent to the free layer on the opposite side from the non-magnetic spacer layer has been proposed in a paper by Y. Kamiguchi et al. (“Co ALLOY SPECULAR SPIN VALVES WITH A NANO OXIDE LAYER” (IEEE, INTERMAG 99, DB-01 (1999)). In this film, conductive electrons are mirror-reflected by the interface between the oxide layer and the metal layer; accordingly, the energy loss of the conductive electrons when the electrons are reflected at the interface is small, and the quantitative ratio of the conductive electrons that are discriminated by the spin-valve effect is large. As a result, the MR ratio is increased and the sensitivity of the sensor is improved, so that a high recording density can be achieved.
However, when an oxide layer that reflects conductive electrons is stacked directly adjacent to the free layer as described in the abovementioned paper, the soft magnetic characteristics deteriorate (e.g., the coercive force of the free layer is increased, etc.) as a result of the diffusion of oxygen from the oxide layer, etc. In the abovementioned paper, the coercive force is actually large, i.e., 14 Oe, so that the sensor is unsuitable as a magnetic sensor. Such a deterioration in the soft magnetic characteristics causes instability in the head output and output asymmetry, as well as base line noise, etc.
On the other hand, in the case of a spin-valve MR sensor in which a non-magnetic back layer is stacked adjacent to the free layer, almost all of the conductive electrons that reach the non-magnetic back layer from the free layer move outside the region of the spin-valve effect created by the change in the relative angle of magnetization of the pinned ferromagnetic layer and the free ferromagnetic layer. If such conductive electrons that are not closely related to the magnetic resistance effect could be utilized, it would appear that a much greater increase in the MR ratio could be achieved.
Accordingly, in view of the abovementioned conventional problems, the ability to provide a spin-valve type magnetoresistance sensor in which the magnetic sensitivity of the sensor can be increased so that a high output can be obtained, and in which instability in the head output and output asymmetry, as well as base line noise, etc., can be suppressed so that the characteristics of the sensor can be stabilized may be beneficial.
Furthermore, the ability to provide a thin-film magnetic head which stably exhibits high performance that can handle a great increase in capacity and an increase in recording density in magnetic recording, and which can also be manufactured at a good yield, as a result of being equipped with such a spin-valve type magnetoresistance sensor may also be beneficial.